Corrosion-resistant alloys

ABSTRACT

An air-meltable, castable, workable, low hardness alloy resistant to sulfuric acid over a wide range of acid concentrations. The alloy consists essentially of between 50.18 and about 55.48% by weight nickel, between 33.20 and about 35.15% by weight chromium, between 3.23 and about 3.85% by weight molybdenum, between about 2.85 and about 3.86% by weight copper, up to 1.50% by weight silicon, up to about 2.50% by weight manganese, up to about 1.00% by weight titanium, up to about 1.00% by weight niobium, up to about 1.00% by weight tantalum, up to about 0.01% by weight boron, up to about 0.30% by weight of a rare earth component selected from the group consisting of cerium, lanthanum and misch metal, and carbon in a proportion between 0 and a maximum which is the greater of 0.08% by weight and the sum of (Ti/5) + (Nb/8)+ (Ta/16) where (Ti) is the titanium content, (Nb) is the niobium content and (Ta) is the tantalum content. The sum of the tantalum content and the niobium content is no greater than about 1.00% by weight. The balance of the alloy is essentially iron.

United States Patent n 1 Culling CORROSION-RESISTANT ALLOYS John H. Culling, Kirkwood, Mo.

[73] Assignee: Carondelet Foundry Company, St.

Louis, Mo.

[22] Filed: Sept. 11, 1974 [21] Appl. No.: 505,005

[75] Inventor:

[56] References Cited UNITED STATES PATENTS 3,759,704 9/l973 Culling 75/! 71 rimary ExaminerR. Dean 'ittorney, Agent, or Firm-Koenig, Senniger, Powers ind Leavitt 57] ABSTRACT \n air-meltable, castable, workable, low hardness [45] July 8,1975

alloy resistant to sulfuric acid over a wide range of acid concentrations. The alloy consists essentially of between 50.18 and about 55.48% by weight nickel, between 33.20 and about 35.15% by weight chromium, between 3.23 and about 3.85% by weight molybdenum, between about 2.85 and about 3.86% by weight copper, up to 1.50% by weight silicon, up to about 2.50% by weight manganese, up to about 1.00% by weight titanium, up to about 1.00% by weight niobium, up to about 1.00% by weight tantalum, up to about 0.01% by weight boron, up to about 0.30% by weight of a rare earth component selected from the group consisting of cerium, lanthanum and misch metal, and carbon in a proportion between 0 and a maximum which is the greater of 0.08% by weight and the sum of [Ti/5] [Nb/81+ [Ta/l6] where [Ti] is the titanium content, [Nb] is the niobium content and [Ta] is the tantalum content. The sum of the tantalum content and the niobium content is no greater than about 1.00% by weight. The balance of the alloy is essentially iron.

6 Claims, No Drawings CORROSION-RESISTANT ALLOYS BACKGROUND OF THE INVENTION This invention relates to corrosion resistant alloys and, more particularly, to weldable, machinable and workable alloys of low hardness which are resistant to corrosion by both oxidizing and reducing sulfuric acid solutions over a wide range of acid strengths.

Sulfuric acid is an ubiquitous industrial reagent which is generally very corrosive to most metals. The corrosivity of sulfuric acid to any given metal, however, varies widely with the strength of the acid, the temperature of the acid environment. and the nature and concentration of various contaminants. Because of the wide ranging uses for sulfuric acid, industrial process streams may be found which run the gamut of sulfuric acid concentrations; which must be handled from temperatures below room temperature up to the boiling point of the acid; and which contain an extensive vari ety of contaminants, for example, other acids and salts,

For purposes of analyzing and predicting their corrosive effect on metals, acids and other corrosive agents are commonly classified as either oxidizing" or reducing." A reducing medium is generally defined as one which includes no component more oxidizing than the hydrogen ion or hydronium ion while an oxidizing medium is one which does contain such a component. Sulfuric acid, along with such other common materials as hydrochloric acid, acetic acid, phosphoric acid, aluminum chloride, hydrobromic acid, and hydrofluoric acid, is normally a reducing medium. At concentrations above approximately 85% by weight, however, sulfuric acid becomes an oxidizing agent. If its temperature is elevated, sulfuric acid may be oxidizing at even lower concentrations. Thus, a 60% by weight sulfuric acid solution becomes oxidizing at temperatures in excess of l50F. Even lower concentrations of sulfuric acid can be moderately to strongly oxidizing when they contain various oxidizing acids and salts, Among the most common solutions of this type are the so-called mixed acids", which are mixtures of sulfuric acid and nitric acid used in organic nitration processes. Other oxidizing materials, some of which may be found in industrial sulfuric acid streams, include hydrogen peroxide, ferric sulfate, silver nitrate, potassum nitrate, sodium nitrate, copper sulfate, potassium permanganate, sodium dichromate, chromic acid, calcium chloride, mercuric chloride, aqua regia, sodium, sodium hypochlorite, ferric chloride, and cupric chloride.

Because of this variety in the character of various industrial sulfuric acid streams, there are relatively few alloys available which can be said to be generally useful in sulfuric acid service. For example, nickel, chromium, molybdenum and copper have all been recognized as useful constituents of alloys designed for exposure to sulfuric acid, yet many combinations of these elements which quite satisfactorily resist the corrosive effect of reducing type sulfuric acid solutions may fail rapidly if a small proportion of an oxidizing agent is present. Other alloys of the same elements may be more resistant to oxidizing media but fail rapidly in a reducing acid. Alloys having other combinations of proportions may be resistant at ambient temperature but fail rapidly at elevated temperatures.

Many alloys which resist dilute sulfuric acid solutions are completely unsuitable for sulfuric acid solutions having concentrations in excess of 60% or 70% by weight. Many other alloys, in particular those containing 5-2371 nickel and 8l8% chromium, demonstrate excellent resistance to sulfuric acid solutions of less than 25% or greater than strength, but fail rapidly in solutions of intermediate concentration. Certain other alloys are available which are highly resistant to a wide range of sulfuric acid solutions, including concentrated sulfuric acid but, for the most part, broadly resistant alloys have been relatively expensive (for example, that sold under the trade designation Illium 98" by Stainless Foundry and Engineering Company, Inc.) or have suffered from undesirable mechanical or other properties often due to the presence of large proportions of silicon. Common drawbacks of such alloys have been poor machinability and weldability, with poor workability being an almost universal problem, i.e., few of these alloys can be feasibly produced in wrought form.

The alloys described in my US. Pat. No. 3,759,704 provide an important improvement over alloys previously available for sulfuric acid service. The alloys described in that patent are resistant to a wide range of sulfuric acid solution concentrations in both the oxidizing and reducing ranges, and many of the alloys de scribed in that patent may be welded, machined and worked. However, those alloys have relatively high hardnesses in the range of l62-24O Brinell hardness, depending on the exact composition and heat treatment. A continuing need has, therefore, existed for alloys having the superior corrosion properties of those described in U.S. Pat. No. 3,759,704, yet possessing lower hardnesses and correspondingly higher elongations for improved adaptability to welding, machining and working.

SUMMARY OF THE INVENTION It is an object of the present invention to provide novel alloys which are resistant to sulfuric acid solutions over a wide range of concentrations. A further object of the present invention is the provision of such alloys which are resistant to sulfuric acid at elevated temperatures. Another object of the present invention is the provision of such alloys which are resistant to sulfuric acid solutions containing oxidizing contaminants. It is a particular object of the present invention to provide such alloys which have a low hardness and are, thus, highly susceptible to welding, machining and working. Other objects and features will be in part apparent and in part pointed out hereinafter.

The present invention is, therefore, directed to an alloy resistant to corrosion by both oxidizing and re ducing sulfuric acid solutions over a wide range of acid strengths. The alloy consists essentially of between about 50.l8 and about 55.48% by weight nickel, between about 33.20 and about 35.15% by weight chromium, between about 3.23 and about 3.85% by weight molybdenum, between about 2.85 and about 3.86% by weight copper, up to about 1.50% by weight silicon, up to about 2.50% by weight manganese, up to about l.00% by weight titanium, up to about 1.00% by weight niobium, up to about l.00% by weight tantalum, up to about 0.01% by weight boron, up to about 0.30% by weight of a rare earth component selected from the group consisting of cerium, lanthanum and misch metal, carbon in a proportion between 0 and a maximum which is the greater of 0.08% by weight and the sum of [Til/5 [Nb]/8 [Ta]/l6 where [Ti] is the titanium content, [Nb] is the niobium content, and [Ta] is the tantalum content. and the balance essentially iron. The sum of the tantalum content and the niobium content is no greater than about 1.00% by weight.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The alloys of the present invention, like those of U.Sv Pat. No. 3,759,704, are highly resistant to corrosion by sulfuric acid solutions over a wide range of compositions. These alloys are resistant to both oxidizing and reducing sulfuric acids and are suitable for use at elevated temperatures. The fabricability properties of the instant alloys, however, are improved over the alloys of U.S. Pat. No. 3,759,704. The alloys of the present invention exhibit substantially lower hardnesses, for example, in the range of 120-130 Brinell Hardness as compared to the alloys of the aforesaid patent which range typically from l60240 Brinell Hardness. As a consequence, the alloys of this invention exhibit high ductility or tensile elongation and are exceptionally well adapted to all fabricating techniques, including welding, machining, forging and other methods of working.

The essential components of the alloys of this invention are:

Nickel 5018-55487: Chromium Bill-35.15% Molybdenum 3.23-3.85Ff Copper LBS-3.86% Iron to make 100% The significantly higher nickel content and carefully controlled proportions of chromium, molybdenum and copper impart iow hardness values and high tensile elongation or ductility values to these alloys.

Optionally, the alloys of the invention may also include carbon, silicon, manganese, titanium, niobium,

attack. The presence of about 0.4 to about 0.6% by weight of either titanium or niobium is, nonetheless, preferred for purposes of enhancing the workability of the alloy.

Further contribution to workability is afforded by very minor proportions of boron up to about 0.01% by weight. Higher proportions of boron should be avoided because of potentially adverse effect on corrosive propertles.

Up to about 2.50% by weight manganese and up to about 1.50% by weight silicon may be advantageously included as deoxidizers and aids to castability of the alloys of the invention. For maximum workability and minimum hardness, however, the silicon content is preferably no higher than about 0.75% by weight. Only very low proportions of manganese and silicon are ever necessary in these alloys since molybdenum, copper, chromium and nickel all provide excellent cleanliness and castability in the ranges of proportions above specified. Often the desired proportions of silicon and manganese may be present as impurities in commonly used melting ingredients.

Optionally, cerium, lanthanum or misch metal may be included to enchance workability of the alloys. Up to a total of about 0.30% by weight cerium, lanthanum and/or misch metal may be included.

The alloys of the invention are prepared by conventional methods of melting and no special conditions such as controlled atmospheres or protective slags are required. In preparing the alloys, the constituents of a melting furnace charge need not be of any particular type. Thus, raw materials such as remelt scrap, copper scrap, ferro alloys such as ferromanganese, and other commercial melting alloys may be used.

The following examples illustrate the invention.

EXAMPLE 1 One hundred-pound heats of two different alloys were prepared in accordance with the invention. Each tantalum and boron. of these heats was then melted in a 100 lb. high fre- TABLE I n PERCENTAGE BY WEIGHT OF ALLOYING ELEMENTS A by No. C Si Mn Ni Cr Mo Cu Fe By Difference Tantalum, titanium and niobium all serve as carbon stabilizers which inhibit the intergranular corrosive attack which may otherwise result from the presence of excessive quantities of carbon. Five parts by weight of titanium stabilizes up to one part by weight carbon, 8 parts by weight niobium stabilizes up to one part by weight carbon, and 16 parts by weight tantalum stabilizes up to one part by weight carbon. Thus, where the maximum proportions of titanium and niobium are present in the alloy, the carbon content may be as high as 0.325% by weight. If the carbon content is maintained at about 0.08% by weight or lower, however, intergranular corrosion can be prevented by proper solution heat treatment and quenching, without the necessity of adding titanium, tantalum or niobium. If the carbon content is about 0.03% by weight or less, moreover, neither heat treatment nor titanium/tantalum/niobium additions are necessary to prevent intergranular TABLE II Tensile Yield Alloy Strength Strength Percent Brinell No. PS1 PS1 Elongation Hardness 1071 61,210 34,400 46.4 l2l l l7l 62,050 36,500 30.5 [28 The corrosion test bars were also annealed for 30 minutes at 1,950F. and oil quenched prior to machining into I V2 inch diameter by one-fourth inch high discs having a one-eighth inch diameter hole in the center. Twelve to l4 discs were obtained for each alloy.

These discs were used in the comparative corrosion mula. in accordance with ASTM specification Gl-67.

Ripy 0.3937 -t where tests described hereinafter comparing the performance 5 corrosion rate in inches per year of the alloys of the invention with two prominent com- Wu Original] weight f sample mercially available alloys. The compositions of the w fi weight f sample commercially available alloys which were used in these A area f sample i Square centimeters tests, their physical properties in the as-cast state, and T duration f test i years the respective trade designations under which they are m D d i f l| i marketed are Set forth in Ta II- Results of this corrosion test are set forth in Table IV.

TABLE III COMPOSITION AND PHYSICAL PROPERTIES OF COMMERCIALLY AVAILABLE ALLOYS IN AS CAST STATE Tensile Yield Stren th Stren h Percent Brinell Alloy Name C% Si7z Mn% Ni% Cr7r Mo% Cu% Fe%"' PS Elongation Hardness Illium 98 0.05 0.79 L25 55.0 28.0 8.5 5.5 l 54,000 41,000 18.0 I60 Carpenter CB3 0.05 0.70 L40 34.0 20.0 2.5 3.5 37 69,000 3 L500 36.0 I55 By Difference TABLE IV CORROSION RATES AS LOSSES IN INCHES OF PENETRATION PER YEAR FOR VARIOUS SULFURIC ACID-WATER SOLUTIONS AT 80C.

Alloy No. 10% 40% 50% 60% 70% 93% 96% or Name H. .so, H 80, n so, H 80, H 80, H50, H50, H 50,

1071 0.0027 0.0030 0.0043 0.00I9 0.0005 0.0008 0.0040 0.0019 I I7] 0.0022 0.0032 0.0030 0.00l6 0.0003 0.0068 0.0038 0.0013 Illium 98 0.0030 0.0057 0.0048 0.0050 0.0044 0.0I07 0.0040 0.0033 Car enter 28Cb3 0.004l 0.0102 0.0091 0.0083 0.0l02 0.0512 0.0202 0.0!73

EXAMPLE 2 In view of the above, it will be seen that the several Comparative corrosion tests were run in IO%, 25%, 40%, 50%, 60%, 70%, 93% and 96% sulfuric acid solutions at 80C.

Disc samples of lllium 98 and Carpenter 20Cb3 were prepared having the same dimensions as the discs prepared in Example 1. Residual machining oil and dirt were removed from all of the sample discs by cleaning with a small amount of carbon tetrachloride. The discs were then rinsed in water and dried. Each disc was weighed to the nearest ten-thousandth of a gram and then suspended in a beaker by a piece of thin platinum wire hooked through the center hole of the disc and attached to a glass rod which rested on the top of the beaker. Sufficient sulfuric acid solution was then added to the beaker so that the entire sample was surrounded. The temperature of the acid was thermostatically controlled at 80C. by means ofa water bath and each beaker was covered with a watch glass to minimize evaporation.

After precisely six hours, the sample discs were removed from the sulfuric acid solution and cleaned of corrosion products. Most samples were cleaned sufficiently with a small nylon bristle brush and tap water. Those samples on which the corrosion product was too heavy for removal with a nylon brush were cleaned with a one-to-one solution of hydrochloric acid and water. After the corrosion products had been removed, each disc was again weighed to the nearest tenthousandth of a gram. The corrosion rate of each disc, in inches per year, was calculated by the following forobjects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above products without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An air-meltable, castable, workable low hardness alloy resistant to corrosion by sulfuric acid over a wide range of acid concentrations consisting essentially of between about 50.18 and about 55.48% by weight nickel, between about 33.20 and about 35.15% by weight chromium, between about 3.23 and about 3.85% by weight molybdenum, between about 2.85 and about 3.86% by weight copper, up to about l.50% by weight silicon, up to about 2.50% by weight manganese, up to about 1.00% by weight titanium, up to about 1.00% by weight niobium, up to about l.00% by weight tantalum, up to about 0.0l% by weight boron, up to about 0.30% by weight of a rare earth component selected from the group consisting of cerium, lanthanum and misch metal, carbon in a proportion between 0 and a maximum which is the greater of 0.08% by weight and the sum of [Ti] [Nb] [Ta] where [Ti] the titanium content [Nb] the niobium content [Ta] tantalum content the sum of the tantalum content and the niobium content being no greater than about l.00% by weight, and the balance essentially iron.

2. An alloy as set forth in claim 1 having a titanium content of between about 0.4 and about 0.6% by weight.

3. An alloy as set forth in claim 1 having a niobium content of between about 0.4 and about 0.6% by weight.

4. An alloy as set forth in claim 1 having a silicon content no greater than about 0.75% by weight.

5. An alloy as set forth in claim 1 comprising approximately 55.48% by weight nickel, approximately 33.20% by weight chromium, approximately 3.23% by weight molybdenum, approximately 2.85% by weight copper, approximately 0.31% by weight silicon. approximately l.4l% by weight manganese, approximately 0.05% carbon and approximately 3.5% by weight iron.

6. An alloy as set forth in claim 1 comprising approximately 50.l8% by weight nickel, approximately 35.15% by weight chromium, approximately 3.85% by weight molybdenum, approximately 3.86% by weight copper, approximately 0.71% by weight silicon, approximately 0.3l% by weight manganese, approximately 0.08% by weight carbon and approximately 5.9% by weight iron. 

1. AN AIR-MELTABLE, CASTABLE, WORKABLE LOW HARDNESS ALLOY RESISTANT TO CORROSION BY SULFURIC ACID OVER A WIDE RANGE OF ACID CONCENTRATIONS CONSISTING ESSENTIALLY OF BETWEEN ABOUT 50.18 AND ABOUT 55.48% BY WEIGHT NICKEL, BETWEEN ABOUT 33.20 AND ABOUT 35.15% BY WEIGHT CHROMIUM, BETWEEN ABOUT 3.23 AND ABOUT 3.85% BY WEIGHT MOLYBDENUM, BETWEEN ABOUT 2.85 AND ABOUT 3.86% BY WEIGHT COPPER, UP TO ABOUT 1.50% BY WEIGHT SILICON, UP TO ABOUT 2.50% BY WEIGHT MANGANESE, UP TO ABOUT 1.00% BY WEIGHT TITANIUM, UP TO ABOUT 1.00% BY WEIGHT NIOBIUM, UP TO ABOUT 1.00% BY WEIGHT TANTALUM, UP TO ABOUT 0.01% BY WEIGHT BORON, UP TO ABOUT 0.30% BY WEIGHT OF A RARE EARTH COMPONENT SELECTED FROM THE GROUP CONSISTING OF CERIUM, LANTHANUM AND MISCH METAL, CARBON IN A PROPORTION BETWEEN 0 AND A MAXIMUM WHICH IS THE GREATER OF 0.08% BY WEIGHT AND THE SUM OF (TI)/5 + (NB)/8 + (TA)/16 WHERE (TI) = THE TITANIUM CONTENT (NB) = THE NIOBIUM CONTENT (TA) = TANTALUM CONTENT THE SUM OF THE TANTALUM CONTENT AND THE NIOBIUM CONTENT BEING NO GREATER THAN ABOUT 1.00% BY EIGHT, AND THE BALANCE ESSENTIALLY IRON.
 2. An alloy as set forth in claim 1 having a titanium content of between about 0.4 and about 0.6% by weight.
 3. An alloy as set forth in claim 1 having a niobium content of between about 0.4 and about 0.6% by weight.
 4. An alloy as set forth in claim 1 having a silicon content no greater than about 0.75% by weight.
 5. An alloy as set forth in claim 1 comprising approximately 55.48% by weight nickel, approximately 33.20% by weight chromium, approximately 3.23% by weight molybdenum, approximately 2.85% by weight copper, approximately 0.31% by weight silicon, approximately 1.41% by weight manganese, approximately 0.05% carbon and approximately 3.5% by weight iron.
 6. An alloy as set forth in claim 1 comprising approximately 50.18% by weight nickel, approximately 35.15% by weight chromium, approximately 3.85% by weight molybdenum, approximately 3.86% by weight copper, approximately 0.71% by weight silicon, approximately 0.31% by weight manganese, approximately 0.08% by weight carbon and approximately 5.9% by weight iron. 